Relatively pure oxygen (i.e. an oxygen-containing gas having an oxygen content of 88% or more) has a number of desirable industrial and medicinal applications at various pressures and purities. The Earth's atmosphere, typically comprising nearly twenty one percent oxygen gas, is the natural candidate for use as an economical oxygen source. As a result, many of the most practical and economical oxygen production plants employ air separation systems and methods.
One of the more common systems for producing oxygen in relatively large volumes incorporates cryogenic technology to liquefy and separate a desired oxygen component of a predetermined purity from the air mixture. While the design works well for high-volume oxygen production, the specialized cryogenic hardware and associated high capital startup expenditures make such systems cost-prohibitive when used for production in low to moderate volumes e.g. from about 30 to about 200 tons per day of an oxygen containing gas with an oxygen concentration higher than about 88% and up to about 95%.
Traditionally, higher volumes of oxygen have been produced via the well-known cryogenic rectification of air in which air is cooled to temperature near the normal boiling point of the components and treated in fractionation columns. The significant capital and operating costs of the cryogenic separation systems are justified only when large quantities and/or extremely high purities (such as 97%-99.999%) are required.
As an alternative to cryogenic processes, those skilled in the art have developed an air separation system that utilizes a molecular sieve adsorbent to efficiently produce oxygen at purities typically ranging from approximately 88 to 93% and up to about 95%. Used in PSA and VPSA systems, the adsorbent more selectively adsorbs N.sub.2 due to the greater quadropole moment of N.sub.2 compared to O.sub.2 to effect component separation.
Adiabatic pressure swing processes are usually accompanied by a thermal cycling ox adverse thermal swing, i.e. the adsorption step occurs at a higher temperature than the desorption step. This thermal swing tends to increase with increasing adsorbate/adsorbent heat of adsorption and may increase with the ratio of adsorption to desorption pressure. In addition, thermal gradients develop within the bed. These gradients and swings in bed temperature result in various parts of the adsorbent bed functioning at different temperatures. The net effect of these gradients and swings in temperature is an overall lower process performance. Adsorbent properties that vary strongly with temperature are also likely to result in process instability when operating conditions change, e.g. normal ambient temperature fluctuations.
The adsorbent is often the key to the effectiveness of the process. Much attention has been given to the development, improvement and manufacture of adsorbents, e.g. specialized zeolite adsorbents have been synthesized through ion exchange, lower Si/Al structures and improved activation procedures. These additional and/or improved manufacturing steps have resulted in higher costs for these specialized adsorbents compared to standard adsorbents, e.g. LiX compared to 5A and 13X adsorbents in air separation. In many processes the adsorbent has become a significant fraction of the overall capital investment. Thus, there is considerable incentive to reduce the cost of the adsorbent if such reduction can be transformed into an overall reduction in the cost of the desired product of the separation.
The prior art has attempted to address the problem of thermal cycling in PSA processes, in some instances by employing mixtures of materials. Mixtures have also been applied independent of thermal cycling effects to improve specific elements of adsorption process performance such as product purity or recovery or storage capacity. Distinct materials have been combined physically (co-mixture) in an adsorber or have been integrally bound in a single composite bead or pellet.
Mixtures of adsorbents have also been utilized when multiple separations are required. An example is provided by Jones et al. (U.S. Pat. No. 4,194,892) for the purification of steam reformer hydrogen involving the removal of carbon dioxide, methane and carbon monoxide using a rapid pressure swing adsorption (RPSA) process. It was shown that product H.sub.2 recovery was increased when a homogeneous mixture of activated carbon and crystalline molecular sieve was used in place of activated carbon alone.
Mixtures of fine and coarse particles have been applied to reduce interparticle void space, increase adsorbent density and increase gas storage capacity. Kaplan et al. (E.P. Pat. Appl. 0 325 392) provides an example of this methodology applied in PSA systems employing carbon molecular sieve (CMS) adsorbents for kinetic separation of air to produce N.sub.2. In Kaplan, the main CMS adsorbent is comprised of coarse particles (2.5 to 3.0 mm) while the void space between these larger particles is filled with fine particles (40-60 mesh) of either an inert material or CMS adsorbent. The fine particle fraction is preferred to be an inert or non-adsorptive materials (e.g. glass beads) and to occupy approximately 40% by volume of the adsorber bed. The reduction in void space was shown to improve process efficiency.
Fuderer (U.S. Pat. No. 4,499,208) doped activated carbon with inert dense alumina and achieved a reduced thermal swing when adsorbing CO.sub.2 at high pressure from a feed stream containing H.sub.2, CO.sub.2, CO and CH.sub.4. Although the specific heat of the alumina is nearly the same as the activated carbon, the high density of the inert material significantly increases the heat capacity per unit volume of the bed. Lowering the thermal swing in the process significantly improved the process recovery.
Mixing high heat capacity inert additives (iron particles) with the adsorbent in the bed to increase the mean heat capacity of the bed was also suggested by Yang (Gas Separation by Adsorption Processes, (pp. 257, 327, 1987).
Gaffney, et. al. (U.S. Pat. No. 5,258,060) used additional binder or an inert diluent to reduce the specific nitrogen capacity of an adsorption zone containing LiX. The inert diluent is preferably of lower heat capacity than the adsorbent and is distributed homogeneously in the bed, either in a composite particle (having increased binder) or as separate particles. The inert diluents comprise from 5% to 80% of the adsorbent bed. This dilution reduces the thermal swing and results in an increase in N.sub.2 capacity and O.sub.2 product recovery.
A mixture of adsorbent and catalyst particles is contemplated in processes combining reaction and separation in a pressure swing reactor (PSR) (Alpay et al., Chem. Eng. Sci. 49, 5845-5864). This disclosure considered mixtures of various adsorbents with a Pt--Al.sub.2 O.sub.3 catalyst in three different industrial reaction schemes of interest. The results suggest improvements in conversion efficiency using the PSR compared to conventional steady flow reactors.
Walter in Ger. Pat. No. P4,443,191 teaches reducing thermal swing by using a single vessel, with multiple internal compartments, each containing adsorbent. The compartments are in thermal contact and arranged so that adjacent compartments are in adsorption and desorption simultaneously. Heat is transferred from the adsorbing compartments to the desorbing compartments. This resulted in increased working capacity.
Savage in U.S. Pat. No. 4,283,204 discloses the use of an adsorbent particle which contains a magnetizable component. A magnetic field is placed across the bed which stabilizes the adsorbent and prevents fluidization. No mention is made of the heat transfer effects between the adsorbent and the magnetic particles. The adsorption and desorption steps are carried out at the same pressure.
Toussaint (U.S. Pat. No. 5,203,887) suggests a reduction in the cost of adsorbent by substituting a layer of less costly NaX for the expensive LiX at the product end of a bed used in air separation processes. A second layer of NaX can also be incorporated at the feed end of the adsorber.
Gas purification, more specifically air prepurification, represents another class of adsorption separation processes where multiple adsorbents can be applied to improve process performance. The operation of cryogenic air separation plants requires large quantities of pretreated air. To prevent freezing and plugging of the primary heat exchanger, the concentration of contaminants such as CO.sub.2 and H.sub.2 O must be lowered to less than 1 ppm. In addition, the concentration of light hydrocarbons which have a low solubility in cryogenic liquids, such as acetylene and certain C.sub.3 -C.sub.8 hydrocarbons, must be kept very low, typically less than 1 ppb, to prevent accumulation within the cryogenic distillation system. Currently both Thermal Swing Adsorption (TSA) and pressure swing adsorption (PSA) are used in air prepurification applications.
TSA prepurifiers use a relatively small amount of heated purge gas to regenerate the adsorption beds. The typical purge to feed ratio is .ltoreq.15%. TSA units are extremely effective at removing the major contaminants such as CO.sub.2, H.sub.2 O and most of the hydrocarbons from an air feed because such adsorbers usually employ strong adsorbents. Any CO and H.sub.2 contained in the feed is generally carried over into the product. If it is necessary to remove the CO and H.sub.2, a sequential oxidation of the CO and H.sub.2 is carried out by catalytic conversion. The strong adsorbents used in TSA processes, such as 5A or 13X zeolite, require the large thermal driving forces available by TSA to affect adequate desorption. The operating adsorbate loadings and selectivities of the major contaminants on these strong adsorbents is such that CO.sub.2 breaks through into the product stream before acetylene and most other hydrocarbons that are harmful to cryogenic air separation plant operation, e.g. C.sub.3 through C.sub.8 hydrocarbons.
The feed gas is usually chilled to minimize the water content of the feed, which in turn reduces the amount of adsorbent required. While the TSA process results in a relatively low purge-to-feed ratio, the inherent heating of the purge and chilling of the feed adds to both the capital and operating cost of the process.
PSA prepurifiers use a near-ambient temperature purge to regenerate the adsorption beds. The reduced driving force that is available from pressure swing alone requires a weaker adsorbent (e.g. alumina), shorter cycles and higher purge-to-feed ratios compared to TSA processes in order to achieve adequate desorption of H.sub.2 O and CO.sub.2 contaminants. Typical purge-to-feed ratios are 40%-60% in PSA prepurification.
The operating loadings of H.sub.2 O and CO.sub.2 on the weak adsorbents used in PSA may actually be larger than those for strong zeolites. Unfortunately, weak adsorbents such as activated alumina are unable to sufficiently retain light hydrocarbons such as acetylene in a reasonable size bed and C.sub.2 H.sub.2 breaks through into the product stream ahead of CO.sub.2. This leads to a potentially hazardous operating condition in a cryogenic air separation process. While the capital costs associated with a PSA prepurifier are lower than those of a TSA, the overall power requirement can be higher. In particular, blowdown or depressurization losses increase power consumption in the PSA prepurifiers, i.e. PSA units cycle much faster than TSA units, resulting in an increase in the frequency of blowdown loss steps.
In light of the above considerations, there is a need in the prepurification art for a PSA adsorbent bed that possesses the favorable desorption characteristics of activated alumina and yet has the acetylene selectivity and loading associated with the stronger zeolites. In addition, there is a need to minimize blowdown losses in order to reduce operating power. The prior art has attempted to address some of these problems.
Hitachi, in German patent application 3045451, discloses a two bed adsorbent system. The first adsorbent (13X) is used to adsorb high concentrations of both H.sub.2 O and CO.sub.2, thus suppressing the coadsorption of nitrogen. The second adsorbent (activated alumina) does not coadsorb nitrogen very strongly. The alumina is used to complete the H.sub.2 O and CO.sub.2 adsorption. By minimizing the nitrogen coadsorption in the beds, blowdown losses during depressurization are likewise minimized. Removal of light hydrocarbons was not addressed.
Kumar, in U.S. Pat. No. 4,711,645, describes a PSA prepurifier which uses activated alumina to adsorb H.sub.2 O and 13X to adsorb CO.sub.2. The use of activated alumina to adsorb H.sub.2 O results in a lower temperature rise in the feed than if 13X were used for the whole bed. This increases the effective capacity of the 13X zone to adsorb CO.sub.2. Other zeolites suggested by Kumar for the second zone are SA, CaA, CaX and Na-mordenite. Removal of light hydrocarbons was not addressed.
Jain, in U.S. Pat. No. 5,232,474 also uses a layer of activated alumina followed by a layer of 13X. Here it is claimed that the activated alumina layer is used to adsorb all the H.sub.2 O and the majority of the CO.sub.2. The purpose of the downstream 13X layer is to remove hydrocarbons and residual CO.sub.2 from the gas stream. Jain teaches that the 13X layer is not intended to remove large amounts of CO.sub.2.
In addition to the prior art cited above that relates to bulk gas separation or air prepurification processes, the prior art also offers several different methods of deployment of material mixtures, e.g. physically mixing at least two different materials, chemically bonding at least two different materials integrally in bead, pellet or granular form, and chemically bonding in preformed structures. Examples of simple physical mixtures of individual materials have already been cited above. The bonding of different materials in a single adsorbent particle or preformed structure typically involves steps of wet mixing, curing, drying and activation. The final composite product may perform better than the average of its individual components. This performance enhancement has not always been well understood, but such improvements have often been attributed to increased surface area and/or activity resulting from the processing of the mixture. In essence, these mixtures or composites represent a new adsorbent with improved physical properties.
Frigert (U.S. Pat. No. 3,025,233) suggests integral porous cores, or structured adsorbents, for the filtration, drying and purification of refrigeration fluids. Zeolite, activated alumina and inert binder may be combined in various ratios in a porous shaped core.
Chi et al. (U.S. Pat. No. 3,899,310) combined active alumina and zeolite to form a composite adsorbent for adsorption of fatty acid compounds from refrigerant gases. The adsorption capacity of the composite was double that of a simple admixture of the same adsorbents. Chi hypothesized that the active surface area of the composite was greater than that of the adsorbent components.
Plee (U.S. Pat. No. 5,173,462) prepared a composite adsorbent containing 70%-95% zeolite with 30% to 5% clay binder, where the zeolite fraction was a mixture of &gt;=95% low-silica CaX and &lt;5% type A. The specific processing, activation and drying methodology applied to the composite was considered important to its performance in air separation processes.
Fleming et al. in U.S. Pat. No. 4,762,537 discloses an adsorbent bead composed of 50-95 wt. % alumina and 5-50 wt. % type Y zeolite for adsorption of HCl in the 100 ppm range. The method of producing the adsorbent results in rates and capacities for HCl which are as high as for a pure NaY bead but which have the chemical resistance to HCl of pure activated alumina. No mention is made of the heat transfer effects between the alumina and the NaY during desorption or in the adsorption step which removes HCl from the gas stream.